Slate roofs are appreciated for their aesthetic and durable qualities. Slate is one of the finest roofing materials available and has several advantages over asphalt shingle roofs. For example, state roofing is fireproof, resists hail damage, and often has a service life of 100 years or more. However, slate is a rigid natural stone product which unfortunately can be damaged by stress. Stress can be introduced into slate in several ways, but the most common cause of stress to slate is nails used to attach the slate to the roof deck. With nail installation, the nails need to be fastened so the slate hangs on the nail. If the nail is inserted too tightly, the nail will pinch the slate. On the other hand if the nail is not inserted deep enough, the overlapping piece of slate may crack from the hidden pressure point. Environmental effects on the wood decking and nails may also contribute to the stress. Environmental changes such as swings in temperature and humidity can cause the decking to expand and contract. If the nails are in a bind in this situation, the slate can crack or fall.
Furthermore, slate roofs are quite expensive (typically two to three times more expensive than composition asphalt roofing), and the weight of the slate is quite high compared to composition shingles (which may require additional support for the roof, further adding cost). Slate materials are expensive themselves, so any reduction in the amount of slate necessary for effective roofing would lead to both a decrease in cost and weight of a roof.
A good background for slate roofing and the method for installing the same may be found in the NRCA Roofing and Waterproofing Manual—4th Edition, pp. 1179-1227, that document being incorporated herein by reference. Typical slate roofs are constructed such that a wood roof is first covered with an underlayment layer, typically asphalt felt paper. Overlapping slate courses are then applied with slate covering the roof in two plies except where there is overlap, in which case there are three plies of slate. Through joints should not occur from the slate roof surface to the felt. So using the conventional slate roofing technique, slate tiles must be elongated sufficiently to allow for three-ply overlap (and two plies of slate on the exposed portions of the roof) in order to ensure that water cannot penetrate the roof between the seams between slate tiles. Accordingly, the conventional slate roofing technique requires the use of a great deal of slate material, due to the need for double ply coverage and three-ply overlap for water resistance, greatly increasing the cost and weight of a slate roof.
Slate roofs may be improved by reducing the amount of slate used to create a waterproof roofing surface, and by eliminating the use of nails (or any other penetration or system requiring a hole in the slate) to secure the slate tiles in place on the roof. This may allow for a more durable, but less expensive and heavy, slate roof. Furthermore, the slate roof would be more durable if there was some means of resisting uplift forces generated by winds on the slate tiles. High winds may catch under the leading edge of the slate tiles, applying a lifting force to the slate. In this manner, wind may increase stresses on the slate tiles. In addition, the wind may actually lift the slate tiles, exposing the underlying roof to the elements. Thus, an improved slate roofing system would attach the slate tiles to the rook deck using some means that would resist wind uplift forces, providing a more durable and weather resistant roof.